ASTM E251-92(2014)
(Test Method)Standard Test Methods for Performance Characteristics of Metallic Bonded Resistance Strain Gages
Standard Test Methods for Performance Characteristics of Metallic Bonded Resistance Strain Gages
SIGNIFICANCE AND USE
4.1 Strain gages are the most widely used devices for the determination of materials, properties and for analyzing stresses in structures. However, performance parameters of strain gages are affected by both the materials from which they are made and their geometric design. These test methods detail the minimum information that must accompany strain gages if they are to be used with acceptable accuracy of measurement.
4.2 Most performance parameters of strain gages require mechanical testing that is destructive. Since test gages cannot be used again, it is necessary to treat data statistically and then apply values to the remaining population from the same lot or batch. Failure to acknowledge the resulting uncertainties can have serious repercussions. Resistance measurement is non-destructive and can be made for each gage.
4.3 Properly designed and manufactured strain gages, whose properties have been accurately determined and with appropriate uncertainties applied, represent powerful measurement tools. They can determine small dimensional changes in structures with excellent accuracy, far beyond that of other known devices. It is important to recognize, however, that individual strain gages cannot be calibrated. If calibration and traceability to a standard are required, strain gages should not be employed.
4.4 To be used, strain gages must be bonded to a structure. Good results depend heavily on the materials used to clean the bonding surface, to bond the gage, and to provide a protective coating. Skill of the installer is another major factor in success. Finally, instrumentation systems must be carefully designed to assure that they do not unduly degrade the performance of the gages. In many cases, it is impossible to achieve this goal. If so, allowance must be made when considering accuracy of data. Test conditions can, in some instances, be so severe that error signals from strain gage systems far exceed those from the structural deformations to be mea...
SCOPE
1.1 The purpose of these test methods are to provide uniform test methods for the determination of strain gage performance characteristics. Suggested testing equipment designs are included.
1.2 Test Methods E251 describes methods and procedures for determining five strain gage parameters:
Section
Part I—General Requirements
7
Part II—Resistance at a Reference Temperature
8
Part III—Gage Factor at a Reference Temperature
9
Part IV—Temperature Coefficient of Gage Factor
10
Part V—Transverse Sensitivity
11
Part VI—Thermal Output
12
1.3 Strain gages are very sensitive devices with essentially infinite resolution. Their response to strain, however, is low and great care must be exercised in their use. The performance characteristics identified by these test methods must be known to an acceptable accuracy to obtain meaningful results in field applications.
1.3.1 Strain gage resistance is used to balance instrumentation circuits and to provide a reference value for measurements since all data are related to a change in the gage resistance from a known reference value.
1.3.2 Gage factor is the transfer function of a strain gage. It relates resistance change in the gage and strain to which it is subjected. Accuracy of strain gage data can be no better than the precision of the gage factor.
1.3.3 Changes in gage factor as temperature varies also affect accuracy although to a much lesser degree since variations are usually small.
1.3.4 Transverse sensitivity is a measure of the strain gage's response to strains perpendicular to its measurement axis. Although transverse sensitivity is usually much less than 10 % of the gage factor, large errors can occur if the value is not known with reasonable precision.
1.3.5 Thermal output is the response of a strain gage to temperature changes. Thermal output is an additive (not multiplicative) error. Therefore, it can...
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Designation: E251 − 92 (Reapproved 2014)
Standard Test Methods for
Performance Characteristics of Metallic Bonded Resistance
Strain Gages
This standard is issued under the fixed designation E251; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
INTRODUCTION
The Organization of International Legal Metrology is a treaty organization with approximately 75
member nations. In 1984, OIML issued International Recommendation No. 62, “Performance
Characteristics of Metallic Resistance Strain Gages.” Test Methods E251 has been modified and
expanded to be the United States ofAmerica’s compliant test specification. Throughout this standard
the terms “strain gage” and “gage” are to be understood to represent the longer, but more accurate,
“metallic bonded resistance strain gages.”
1. Scope subjected. Accuracy of strain gage data can be no better than
the precision of the gage factor.
1.1 The purpose of these test methods are to provide
1.3.3 Changes in gage factor as temperature varies also
uniform test methods for the determination of strain gage
affect accuracy although to a much lesser degree since varia-
performance characteristics. Suggested testing equipment de-
signs are included. tions are usually small.
1.3.4 Transversesensitivityisameasureofthestraingage’s
1.2 Test Methods E251 describes methods and procedures
response to strains perpendicular to its measurement axis.
for determining five strain gage parameters:
Although transverse sensitivity is usually much less than 10%
Section
Part I—General Requirements 7 of the gage factor, large errors can occur if the value is not
Part II—Resistance at a Reference Temperature 8
known with reasonable precision.
Part III—Gage Factor at a Reference Temperature 9
1.3.5 Thermal output is the response of a strain gage to
Part IV—Temperature Coefficient of Gage Factor 10
Part V—Transverse Sensitivity 11
temperature changes. Thermal output is an additive (not
Part VI—Thermal Output 12
multiplicative) error. Therefore, it can often be much larger
1.3 Strain gages are very sensitive devices with essentially
than the gage output from structural loading. To correct for
infinite resolution. Their response to strain, however, is low
these effects, thermal output must be determined from gages
and great care must be exercised in their use.The performance
bonded to specimens of the same material on which the tests
characteristics identified by these test methods must be known
are to run, often to the test structure itself.
to an acceptable accuracy to obtain meaningful results in field
1.4 Bonded resistance strain gages differ from extensom-
applications.
eters in that they measure average unit elongation (∆L/L) over
1.3.1 Strain gage resistance is used to balance instrumenta-
tioncircuitsandtoprovideareferencevalueformeasurements a nominal gage length rather than total elongation between
definite gauge points. Practice E83 is not applicable to these
sincealldataarerelatedtoachangeinthegageresistancefrom
a known reference value. gages.
1.3.2 Gage factor is the transfer function of a strain gage. It
1.5 These test methods do not apply to transducers, such as
relates resistance change in the gage and strain to which it is
load cells and extensometers, that use bonded resistance strain
gages as sensing elements.
1.6 strain gages are part of a complex system that includes
These test methods are under the jurisdiction of ASTM Committee E28 on
Mechanical Testing and are the direct responsibility of Subcommittee E28.01 on
structure, adhesive, gage, lead wires, instrumentation, and
Calibration of Mechanical Testing Machines and Apparatus.
(often) environmental protection. As a result, many things
Current edition approved April 15, 2014. Published August 2014. Originally
affect the performance of strain gages, including user tech-
approved in 1964. Last previous edition approved in 2009 as E251–92 (2009).
DOI: 10.1520/E0251-92R14. nique.Afurthercomplicationisthatstraingagesonceinstalled
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E251 − 92 (2014)
normally cannot be reinstalled in another location. Therefore,
R = the strain gage resistance at test strain,
gage characteristics can be stated only on a statistical basis.
R = the strain gage resistance at zero or reference strain,
o
L = the test structure length under the strain gage at test
1.7 This standard does not purport to address all of the
strain,
safety concerns, if any, associated with its use. It is the
L = the test structure length under the strain gage at zero
o
responsibility of the user of this standard to establish appro-
or reference strain,
priate safety and health practices and determine the applica-
∆R = the change in strain gage resistance when strain is
bility of regulatory limitations prior to use.
changed from zero (or reference strain to test strain),
2. Referenced Documents
L2L
2 ε = o
2.1 ASTM Standards:
the mechanical strain .
L
o
E83Practice for Verification and Classification of Exten-
3.2.5 gage length (see Fig. 1)—the length of the strain
someter Systems
sensitive section of a strain gage in the measurement axis
E228Test Method for Linear Thermal Expansion of Solid
direction.
Materials With a Push-Rod Dilatometer
3.2.5.1 Discussion—An approximation of this length is the
E289Test Method for Linear Thermal Expansion of Rigid
distance between the inside of the strain gage end loops. Since
Solids with Interferometry
the true gage length is not known, gage length may be
E1237Guide for Installing Bonded Resistance Strain Gages
measured by other geometries (such as the outside of the end
2.2 Other Standards:
loops) providing that the deviation is defined.
OIML International Recommendation No. 62Performance
3.2.6 grid (see Fig. 1)—that portion of the strain-sensing
Characteristics of Metallic Resistance Strain Gages
material of the strain gage that is primarily responsible for
3. Terminology resistance change due to strain.
3.2.7 lot—a group of strain gages with grid elements from a
3.1 The vocabulary included herein has been chosen so that
common melt, subjected to the same mechanical and thermal
specialized terms in the strain gage field are clearly defined.A
processes during manufacturing.
typical strain gage nomenclature is provided in Appendix X1.
3.2.8 matrix—(see Fig. 1)—an electrically nonconductive
3.2 Definitions of Terms Specific to This Standard:
layer of material used to support a strain gage grid.
3.2.1 batch—a group of strain gages of the same type and
3.2.8.1 Discussion—The two main functions of a matrix are
lot,manufacturedasaset(madeatthesametimeandunderthe
to act as an aid for bonding the strain gage to a structure and
same conditions).
as an electrically insulating layer in cases where the structure
3.2.2 calibration apparatus— equipment for determining a
is electrically conductive.
characteristic of a bonded resistance strain gage by accurately
producing the necessary strains, temperatures, and other con-
ditions; and, by accurately measuring the resulting change of
gage resistance.
3.2.3 error-strain gage— the value obtained by subtracting
the actual value of the strain, determined from the calibration
apparatus, from the indicated value of the strain given by the
strain gage output.
3.2.3.1 Discussion—Errors attributable to measuring sys-
tems are excluded.
3.2.4 gage factor— the ratio between the unit change in
strain gage resistance due to strain and the causing strain.
3.2.4.1 Discussion—Thegagefactorisdimensionlessandis
expressed as follows:
R 2 R ∆R
o
R R
o o
K 5 5 (1)
L 2 L ε
o
L
o
where:
K = the gage factor,
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
Available from OIML International Organization of Legal Metrology, BIML,
11, rue Turgot, F-75009 Paris, France, http://www.oiml.org/en FIG. 1 Typical Strain Gage
E251 − 92 (2014)
3.2.9 measurement axis (grid) (see Fig. 1)—that axis that is the minimum information that must accompany strain gages if
parallel with the grid lines. they are to be used with acceptable accuracy of measurement.
3.2.10 strain gage, metallic, resistive, bonded (see Fig.
4.2 Most performance parameters of strain gages require
1)—a resistive element, with or without a matrix that is
mechanical testing that is destructive. Since test gages cannot
attached to a solid body by cementing, welding, or other
be used again, it is necessary to treat data statistically and then
suitable techniques so that the resistance of the element will
apply values to the remaining population from the same lot or
vary as the surface to which it is attached is deformed.
batch. Failure to acknowledge the resulting uncertainties can
3.2.10.1 Discussion—These test methods apply to gages
have serious repercussions. Resistance measurement is non-
where the instantaneous gage resistance, R, is given by the
destructive and can be made for each gage.
equation:
4.3 Properly designed and manufactured strain gages,
R 5 R ~11εK! (2)
whose properties have been accurately determined and with
o
appropriate uncertainties applied, represent powerful measure-
where:
ment tools. They can determine small dimensional changes in
R = element resistance at reference strain and temperature
o
structures with excellent accuracy, far beyond that of other
levels (frequently initial test or balanced circuit
known devices. It is important to recognize, however, that
conditions),
individual strain gages cannot be calibrated. If calibration and
ε = linear strain of the surface in the direction of the
traceability to a standard are required, strain gages should not
strain-sensitive axis of the gage, and
be employed.
K = a proportionality factor (see gage factor).
4.4 To be used, strain gages must be bonded to a structure.
3.2.11 strain, linear—theunitelongationinducedinaspeci-
Good results depend heavily on the materials used to clean the
men either by a stress field (mechanical strain) or by a
bonding surface, to bond the gage, and to provide a protective
temperature change (thermal expansion).
coating.Skilloftheinstallerisanothermajorfactorinsuccess.
3.2.12 temperature coeffıcient of gage factor—the ratio of
Finally, instrumentation systems must be carefully designed to
the unit variation of gage factor to the temperature variation,
assure that they do not unduly degrade the performance of the
expressed as follows:
gages.Inmanycases,itisimpossibletoachievethisgoal.Ifso,
K 2 K 1
allowance must be made when considering accuracy of data.
t1 t0
· (3)
S D S D
K T 2 T
Test conditions can, in some instances, be so severe that error
t0 1 0
signals from strain gage systems far exceed those from the
where:
structural deformations to be measured. Great care must be
T = the test temperature,
exercised in documenting magnitudes of error signals so that
T = the reference temperature,
realistic values can be placed on associated uncertainties.
K = the gage factor at test temperature, and
t1
K = the gage factor at reference temperature.
t0
5. Interferences
3.2.13 thermal expansion—the dimensional change of an
5.1 To assure that strain gage test data are within a defined
unconstrainedspecimensubjecttoachangeintemperaturethat
accuracy, the gages must be properly bonded and protected
is uniform throughout the material.
with acceptable materials. It is normally simple to ascertain
3.2.14 thermal output—the reversible part of the tempera-
that strain gages are not performing properly. The most
ture induced indicated strain of a strain gage installed on an
common symptom is instability with time or temperature
unrestrained test specimen when exposed to a change in
change.Ifstraingagesdonotreturntotheirzeroreadingwhen
temperature.
theoriginalconditionsarerepeated,orthereisloworchanging
3.2.15 transverse axis (see Fig. 1)—the strain gage axis at resistance to ground, the installation is suspect. Aids in strain
90° to the measurement axis.
gageinstallationandverificationthereofcanbefoundinGuide
E1237.
3.2.16 transverse sensitivity—the ratio, expressed as a
percentage, of the unit change of resistance of a strain gage
6. Hazards
mounted perpendicular to a uniaxial strain field (transverse
gage) to the unit resistance change of a similar gage mounted
6.1 In the specimen surface cleaning, gage bonding, and
parallel to the same strain field (longitudinal gage). protection steps of strain gage installation, hazardous chemi-
cals may be used. Users of these test methods are responsible
3.2.17 type—a group of strain gages that are nominally
for contacting manufacturers of these chemicals for applicable
identical with respect to physical and manufacturing charac-
Material Safety Data Sheets and to adhere to the required
teristics.
precautions.
4. Significance and Use
7. Test Requirements
4.1 Strain gages are the most widely used devices for the
determination of materials, properties and for analyzing 7.1 General Environmental Requirements:
stresses in structures. However, performance parameters of 7.1.1 Ambient Conditions at Room Temperature—The
straingagesareaffectedbyboththematerialsfromwhichthey nominal temperature and relative humidity shall be 23°C
aremadeandtheirgeometricdesign.Thesetestmethodsdetail (73°F) and 50%, respectively. In no case shall the temperature
E251 − 92 (2014)
be less that 18°C (64°F) nor greater than 25°C (77°F) and the where:
relative humidity less than 35% nor more than 60%. The
E = input voltage,
i
fluctuationsduringanyroomtemperaturetestofanygageshall
R = resistance required for initial bridge balance, and
o
not exceed6 2°C and 6 5% RH. ∆R = difference between the instantaneous resistance and
R .
7.1.2 Ambient Conditions at Elevated and Lower o
Temperatures—The temperature adjustment error shall not This circuit is readily adaptable to automatic recording of
exceed 6 2°C (6 3.6°F) or 6 2% of the deviation from room
data. Either ac or dc excitation may be used, but errors due to
temperature, whichever is greater. The total uncertainty of thermal emfs and reactive changes are possible. Loading
temperatureshallnotexceed 62°C(63.6°F),or 61%ofthe effects due to the impedance of the recording instruments may
deviation
...
This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: E251 − 92 (Reapproved 2009) E251 − 92 (Reapproved 2014)
Standard Test Methods for
Performance Characteristics of Metallic Bonded Resistance
Strain GaugesGages
This standard is issued under the fixed designation E251; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
INTRODUCTION
The Organization of International Legal Metrology is a treaty organization with approximately 75
member nations. In 1984, OIML issued International Recommendation No. 62, 'Performance“Perfor-
mance Characteristics of Metallic Resistance Strain Gauges.’Gages.” Test Methods E251 has been
modified and expanded to be the United States of America’s compliant test specification. Throughout
this standard the terms “strain gauge”gage” and “gauge”“gage” are to be understood to represent the
longer, but more accurate, “metallic bonded resistance strain gauges.”gages.”
1. Scope
1.1 The purpose of this standard is these test methods are to provide uniform test methods for the determination of strain
gaugegage performance characteristics. Suggested testing equipment designs are included.
1.2 Test Methods E251 describes methods and procedures for determining five strain gaugegage parameters:
Section
Part I—General Requirements 7
Part II—Resistance at a Reference Temperature 8
Part III—Gauge Factor at a Reference Temperature 9
Part III—Gage Factor at a Reference Temperature 9
Part IV—Temperature Coefficient of Gauge Factor 10
Part IV—Temperature Coefficient of Gage Factor 10
Part V—Transverse Sensitivity 11
Part VI—Thermal Output 12
1.3 Strain gaugesgages are very sensitive devices with essentially infinite resolution. Their response to strain, however, is low
and great care must be exercised in their use. The performance characteristics identified by these test methods must be known to
an acceptable accuracy to obtain meaningful results in field applications.
1.3.1 Strain gaugegage resistance is used to balance instrumentation circuits and to provide a reference value for measurements
since all data are related to a change in the gaugegage resistance from a known reference value.
1.3.2 GaugeGage factor is the transfer function of a strain gauge.gage. It relates resistance change in the gaugegage and strain
to which it is subjected. Accuracy of strain gaugegage data can be no better than the precision of the gaugegage factor.
1.3.3 Changes in gaugegage factor as temperature varies also affect accuracy although to a much lesser degree since variations
are usually small.
1.3.4 Transverse sensitivity is a measure of the strain gauge’sgage’s response to strains perpendicular to its measurement axis.
Although transverse sensitivity is usually much less than 10 % of the gaugegage factor, large errors can occur if the value is not
known with reasonable precision.
1.3.5 Thermal output is the response of a strain gaugegage to temperature changes. Thermal output is an additive (not
multiplicative) error. Therefore, it can often be much larger than the gaugegage output from structural loading. To correct for these
effects, thermal output must be determined from gaugesgages bonded to specimens of the same material on which the tests are to
run;run, often to the test structure itself.
These test methods are under the jurisdiction of ASTM Committee E28 on Mechanical Testing and are the direct responsibility of Subcommittee E28.01 on Calibration
of Mechanical Testing Machines and Apparatus.
Current edition approved April 1, 2009April 15, 2014. Published September 2009August 2014. Originally approved in 1964. Last previous edition approved in 20032009
as E251 – 92 (2003).(2009). DOI: 10.1520/E0251-92R09.10.1520/E0251-92R14.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E251 − 92 (2014)
1.4 Bonded resistance strain gaugesgages differ from extensometers in that they measure average unit elongation (ΔL/L) over
a nominal gaugegage length rather than total elongation between definite gauge points. Practice E83 is not applicable to these
gauges.gages.
1.5 These test methods do not apply to transducers, such as load cells and extensometers, that use bonded resistance strain
gaugesgages as sensing elements.
1.6 Strain gaugesstrain gages are part of a complex system that includes structure, adhesive, gauge, leadwires, gage, lead wires,
instrumentation, and (often) environmental protection. As a result, many things affect the performance of strain gauges,gages,
including user technique. A further complication is that strain gaugesgages once installed normally cannot be reinstalled in another
location. Therefore, gaugegage characteristics can be stated only on a statistical basis.
1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
E83 Practice for Verification and Classification of Extensometer Systems
E228 Test Method for Linear Thermal Expansion of Solid Materials With a Push-Rod Dilatometer
E289 Test Method for Linear Thermal Expansion of Rigid Solids with Interferometry
E1237 Guide for Installing Bonded Resistance Strain Gages
2.2 OIML International Recommendation No. 62:' Performance Characteristics of Metallic Resistance Strain GaugesOther
Standards:
OIML International Recommendation No. 62 Performance Characteristics of Metallic Resistance Strain Gages
3. Terminology
3.1 The vocabulary included herein has been chosen so that specialized terms in the strain gage field are clearly defined. A
typical strain gage nomenclature is provided in Appendix X1.
3.2 Definitions of Terms Specific to This Standard:
3.1.1 The vocabulary included herein has been chosen so that specialized terms in the strain gauge field will be clearly defined.
A typical strain gauge nomenclature is provided in Appendix X1.
3.1.1.1 batch—a group of strain gauges of the same type and lot, manufactured as a set (made at the same time and under the
same conditions).
3.1.1.2 calibration apparatus—equipment for determining a characteristic of a bonded resistance strain gauge by accurately
producing the necessary strains, temperatures, and other conditions; and, by accurately measuring the resulting change of gauge
resistance.
3.1.1.3 error-strain gauge—the value obtained by subtracting the actual value of the strain, determined from the calibration
apparatus, from the indicated value of the strain given by the strain gauge output. Errors attributable to measuring systems are
excluded.
3.1.1.4 gauge factor—the ratio between the unit change in strain gauge resistance due to strain and the causing strain. The gauge
factor is dimensionless and is expressed as follows:
R 2 R L 2 L ΔR
o o
K 5 / 5 /ε (1)
R L R
o o o
where:
K = the gauge factor,
R = the strain gauge resistance at test strain,
R = the strain gauge resistance at zero or reference strain,
o
L = the test structure length under the strain gauge at test strain,
L = the test structure length under the strain gauge at zero or reference strain,
o
ΔR = the change in strain gauge resistance when strain is changed from zero (or reference strain to test strain),
ε = the mechanical strain L2L L .
o o
/
3.1.1.5 gauge length (see Fig. 1)—the length of the strain sensitive section of a strain gauge in the measurement axis direction.
An approximation of this length is the distance between the inside of the strain gauge end loops. Since the true gauge length is
not known, gauge length may be measured by other geometries (such as the outside of the end loops) providing that the deviation
is defined.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Available from OIML International Organization of Legal Metrology, BIML, 11, rue Turgot, F-75009 Paris, France, http://www.oiml.org/en
E251 − 92 (2014)
FIG. 1 Typical Strain GaugeGage
3.1.1.6 grid (see Fig. 1)—that portion of the strain-sensing material of the strain gauge that is primarily responsible for
resistance change due to strain.
3.1.1.7 lot—a group of strain gauges with grid elements from a common melt, subjected to the same mechanical and thermal
processes during manufacturing.
3.1.1.8 matrix (see Fig. 1)—an electrically nonconductive layer of material used to support a strain gauge grid. The two main
functions of a matrix are to act as an aid for bonding the strain gauge to a structure and as an electrically insulating layer in cases
where the structure is electrically conductive.
3.1.1.9 measurement axis (grid) (see Fig. 1)—that axis that is parallel with the grid lines.
3.1.1.10 strain gauge, metallic, resistive, bonded (see Fig. 1)—a resistive element, with or without a matrix that is attached to
a solid body by cementing, welding, or other suitable techniques so that the resistance of the element will vary as the surface to
which it is attached is deformed. These test methods apply to gauges where the instantaneous gauge resistance, R, is given by the
equation:
R 5 R ~11εK! (2)
o
where:
R = element resistance at reference strain and temperature levels (frequently initial test or balanced circuit conditions),
o
ε = linear strain of the surface in the direction of the strain-sensitive axis of the gauge, and
K = a proportionality factor (see gauge factor).
3.1.1.11 strain, linear—the unit elongation induced in a specimen either by a stress field (mechanical strain) or by a temperature
change (thermal expansion).
3.1.1.12 temperature coeffıcient of gauge factor—the ratio of the unit variation of gauge factor to the temperature variation,
expressed as follows:
K 2 K 1
t1 t0
· (3)
S DS D
K T 2 T
t0 1 0
where:
T = the test temperature,
T = the reference temperature,
K = the gauge factor at test temperature, and
t1
K = the gauge factor at reference temperature.
t0
3.1.1.13 thermal expansion—the dimensional change of an unconstrained specimen subject to a change in temperature that is
uniform throughout the material.
E251 − 92 (2014)
3.1.1.14 thermal output—the reversible part of the temperature induced indicated strain of a strain gauge installed on an
unrestrained test specimen when exposed to a change in temperature.
3.1.1.15 transverse axis (see Fig. 1)—the strain gauge axis at 90° to the measurement axis.
3.1.1.16 transverse sensitivity—the ratio, expressed as a percentage, of the unit change of resistance of a strain gauge mounted
perpendicular to a uniaxial strain field (transverse gauge) to the unit resistance change of a similar gauge mounted parallel to the
same strain field (longitudinal gauge).
3.1.1.17 type—a group of strain gauges that are nominally identical with respect to physical and manufacturing characteristics.
3.2.1 batch—a group of strain gages of the same type and lot, manufactured as a set (made at the same time and under the same
conditions).
3.2.2 calibration apparatus— equipment for determining a characteristic of a bonded resistance strain gage by accurately
producing the necessary strains, temperatures, and other conditions; and, by accurately measuring the resulting change of gage
resistance.
3.2.3 error-strain gage— the value obtained by subtracting the actual value of the strain, determined from the calibration
apparatus, from the indicated value of the strain given by the strain gage output.
3.2.3.1 Discussion—
Errors attributable to measuring systems are excluded.
3.2.4 gage factor— the ratio between the unit change in strain gage resistance due to strain and the causing strain.
3.2.4.1 Discussion—
The gage factor is dimensionless and is expressed as follows:
R 2 R ΔR
o
R R
o o
K 5 5 (1)
L 2 L ε
o
L
o
where:
K = the gage factor,
R = the strain gage resistance at test strain,
R = the strain gage resistance at zero or reference strain,
o
L = the test structure length under the strain gage at test strain,
L = the test structure length under the strain gage at zero or reference strain,
o
ΔR = the change in strain gage resistance when strain is changed from zero (or reference strain to test strain),
L2L
ε =
o
the mechanical strain .
L
o
3.2.5 gage length (see Fig. 1)—the length of the strain sensitive section of a strain gage in the measurement axis direction.
3.2.5.1 Discussion—
An approximation of this length is the distance between the inside of the strain gage end loops. Since the true gage length is not
known, gage length may be measured by other geometries (such as the outside of the end loops) providing that the deviation is
defined.
3.2.6 grid (see Fig. 1)—that portion of the strain-sensing material of the strain gage that is primarily responsible for resistance
change due to strain.
3.2.7 lot—a group of strain gages with grid elements from a common melt, subjected to the same mechanical and thermal
processes during manufacturing.
3.2.8 matrix—(see Fig. 1)—an electrically nonconductive layer of material used to support a strain gage grid.
3.2.8.1 Discussion—
The two main functions of a matrix are to act as an aid for bonding the strain gage to a structure and as an electrically insulating
layer in cases where the structure is electrically conductive.
3.2.9 measurement axis (grid) (see Fig. 1)—that axis that is parallel with the grid lines.
E251 − 92 (2014)
3.2.10 strain gage, metallic, resistive, bonded (see Fig. 1)—a resistive element, with or without a matrix that is attached to a
solid body by cementing, welding, or other suitable techniques so that the resistance of the element will vary as the surface to
which it is attached is deformed.
3.2.10.1 Discussion—
These test methods apply to gages where the instantaneous gage resistance, R, is given by the equation:
R 5 R 11εK (2)
~ !
o
where:
R = element resistance at reference strain and
...
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